With the rapid spread of wireless devices typified by cellular phones, small and lightweight resonators and filters made by combining such resonators have been increasingly demanded. Until now, dielectrics and surface acoustic wave (SAW) filters have been mainly used. Hitherto, in recent years, a piezoelectric thin film resonator which is an element in which the characteristics particularly in high frequencies are excellent and which can be reduced in the size and structured into a monolithic, and a filter using such piezoelectric thin film resonator have increasingly attracted attention.
As one of such piezoelectric thin film resonators, a FBAR (Film Bulk Acoustic Resonator) type resonator is known. The FBAR has a multilayered structure (composite film) of an upper electrode, a piezoelectric film, and a lower electrode on a substrate and has a via hole or a cavity (space) in order to prevent the dissipation of vibrational energy to the substrate under the lower electrode in a portion where the upper electrode and the lower electrode face each other. In some cases, a space is formed under the lower electrode through a dielectric film. The via hole can be formed by etching, for example, an Si substrate to be used as an element substrate from the back surface. The cavity can be formed by forming a resonator, such as a composite film, on a sacrificial layer pattern on the substrate surface, and finally removing the sacrificial layer. In the following description, the piezoelectric thin film resonator having the via hole as a space is referred to as a “via hole type,” whereas the piezoelectric thin film resonator having the cavity is referred to as a “cavity type”.
In the piezoelectric thin film resonator configured as described above, when a high frequency electrical signal is applied between the upper electrode and the lower electrode, an elastic wave excited by converse piezoelectric effects or an elastic wave produced by distortion resulting from piezoelectric effects arises in the piezoelectric film interposed between the upper electrode and the lower electrode. The elastic waves are converted to electrical signals. Such elastic waves are converted to thickness longitudinal oscillation waves having the main displacement in the thickness direction because the waves are totally reflected on the surface where the upper electrode and the lower electrode each contact the air. With such element structure, resonance occurs at the frequency where the total film thickness H of the multilayered structure containing the upper electrode, the piezoelectric film, and the lower electrode formed on the space becomes equal to the integral multiple (n times) of the ½ wavelength of the elastic wave. The transmitting velocity V of the elastic wave is decided depending on materials, and the resonance frequency F is determined by:F=nV/2H. 
When such a resonance phenomenon is utilized, the resonance frequency can be controlled by using the film thickness as a parameter, and resonators or filters having desired frequency characteristics can be produced.
As the upper electrode and the lower electrode, metal materials, such as aluminum (Al), copper (Cu), molybdenum (Mo), tungsten (W), tantalum (Ta), platinum (Pt), ruthenium (Ru), rhodium (Rh), iridium (Ir), chromium (Cr), or titanium (Ti) or laminated materials containing a combination of such metals can be used.
As the piezoelectric film, aluminum nitride (AlN), zinc oxide (ZnO), lead zirconate titanate (PZT), lead titanate (PbTiO3), etc., can be used. In particular, aluminum nitride (AlN) and zinc oxide (ZnO) having an orientation axis in the (002) direction during film formation are preferable for the piezoelectric film.
As the substrate, silicon (Si), glass, gallium arsenide (GaAs), and the like can be used.
FIG. 17 is a cross sectional view of a via hole type piezoelectric thin film resonator disclosed in Non-patent Document 1. As shown in FIG. 17, an Au—Cr film as a lower electrode 103, a ZnO film as a piezoelectric film 104, and a an Al film as an upper electrode 105 form a multilayered structure on an Si(100) substrate 101 having a thermal oxidation film (SiO2) 102. Under the multilayered structure, a space (via hole) 106 is formed. The space 106 is formed from the back surface of the Si(100) substrate 101 using anisotropic etching using an aqueous KOH solution or an aqueous EDP solution (mixed liquid of ethylene diamine, pyrocatechol, and water).
FIG. 18 is a cross sectional view of a cavity type piezoelectric thin film resonator disclosed in Patent Document 1. As shown in FIG. 18, a multilayered structure containing a lower electrode 203, a piezoelectric film 204, and an upper electrode 205 is formed on a substrate 201 having a thermal oxidation film (SiO2) 202. Under the multilayered structure, a space (cavity) 206 is formed. The space 206 can be formed by forming an island-shaped ZnO sacrificial layer pattern beforehand on the substrate 201, forming a multilayered structure on the sacrificial layer pattern, and then removing the sacrificial layer present under the multilayered structure with an etching liquid, such as acid.
In the piezoelectric thin film resonator, the orientation properties of the piezoelectric thin film are preferably uniform in order to obtain a high coupling coefficient in a resonance region where the lower electrode and the upper electrode face each other. However, the end of the lower electrode has a level difference or inclination, and thus the orientation properties of the piezoelectric thin film become discontinuous.
The discontinuous region of this piezoelectric film not only degrades the properties of the piezoelectric thin film resonator, but also has serious influence on the reliability.
In order to solve the above-described problems, the structure shown in FIGS. 19 and 20 is disclosed in Patent Document 3. FIG. 19 is a plan view of the piezoelectric thin film resonator disclosed in Patent Document 3. FIG. 20 is a cross sectional view of the Z-Z′ region in FIG. 19. As shown in FIGS. 19 and 20, it is devised that, in a region 307 where a lower electrode 303 and an upper electrode 305 face each other, a discontinuous portion 308 of a piezoelectric film 304 generated due to the level difference at the end of the lower electrode described above is outside the region of a space 306, and the discontinuous portion 308 does not substantially function as a resonator.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 60-189307    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2006-128993    [Patent Document 3] Japanese Unexamined Patent Application Publication No. 2002-140075    [Non-patent Document 1] Electron. Lett., 1981, volume 17, pp. 507 to 509